See https://bugs.musicpd.org/view.php?id=4547

For easier state management inside filter plugins.

While this assert() was useful when we had to track the object's state
manually, there was no practical purpose other than verifying old
code, and it complicates our new C++ code.

The variable is actually used in this function.

This is a left-over from the previous version of the code
that was retrying on EINTR.

Some minor style fixes as well.

This can happen if you remove an external audio card or if you stop
sndiod(8) while playing a song.

sio_write() will retry internally if it fails with errno == EINTR
so no need to handle that.

It defaults to 250 ms.

Fixes build failure on Debian Wheezy.

Use the snd_pcm_t only in the IOThread, and reuse code that is
well-known to work.

Comes with a callback pointer instead of a virtual method, which
allows multiple instances in one class.

Replaces the old BoundMethod template.